1. Field of the Invention
The present invention relates to a polishing apparatus, and relates in particular to a polishing apparatus having a top ring or pedestal having a rotation shaft is supported by a magnetic bearing.
2. Description of the Prior Art
In recent years, there has been a remarkable progress in the density of integrated circuit devices leading to a trend of narrowing interline spacing. In the case of using an optical lithography process involving less than 0.5 .mu.m line spacing particularly, the shallow depth of focus is associated to demand that the focusing plane of the stepper device be highly flat. Furthermore, if there is a particle of a size larger than the interline spacing, problems of electrical short circuiting can occur. Therefore, both flatness and cleanliness are important considerations in device fabrication. Such considerations apply equally to glass substrates used in masking or to liquid crystal display panels.
A conventional polishing apparatus, shown in FIG. 19, comprises a turntable 2 having a polishing device (polishing pad) 1 mounted on the top surface and a top ring 3 holding an object to be polished such as semiconductor wafer W. The turntable 2 and the top ring 3 are provided with their own rotation drivers so as to be independently rotated, along with a pressing device such as an air cylinder to press the wafer onto the polishing pad.
Such a polishing apparatus is operated by placing the top ring 3 in such a way that the edge of a wafer W is positioned at a certain distance away from the center and the edge of the turntable 2, and by pressing the wafer W towards the turntable while rotating the turntable 2 and the top ring 3 at independent speeds and supplying a polishing solution Q from a nozzle 4. The aim is to polish the entire surface of the wafer W uniformly.
The top ring 3 is a disc shaped member holding object W on its underside holding surface by suction, for example, and it is attached to its support shaft 5 at a joint section providing universal coupling by means of a sphere 6. This structural arrangement allows tilting of the top ring 3 in response to the force exerted by the turntable 2 so as to compensate for any misalignment between the top ring 3 and the turntable 2 or local variations in the polishing pad 1 so that polishing can be carried out consistently.
However, such a conventional polishing technique presents a problem that a pressing pressure between the abrading surface (pad surface) of the turntable 2 and the object W tends to be non-uniform across the surface so that it is difficult to obtain a uniform removal rate across the surface of the object W. This will be examined more closely below in terms of the relative motions of the object W held in the top ring 3 and the polishing pad 1 on the turntable 2.
The amount of material removed by polishing is given by a relation: EQU Qp=.eta..times.P.times.V.times.T
where Qp is removal rate; .eta.is a constant; P is a polishing pressure; V is a relative speed (between object surface of the wafer and the turntable); and T is a polishing duration. Therefore, applying uniform pressure in the polishing area is one of the important factors to obtain a uniform removal rate within the polished surface.
However, as shown in a schematic drawing presented in FIG. 20A, there is a force of friction "f" acting at the polish surface (given by f=mN where N is the load on the object W and m is the coefficient of friction) which generates a rotational moment M around the sphere 6. This arrangement produces tilting of the top ring 3, as illustrated in FIG. 20B, resulting in a phenomenon of "plunging" of the leading edge portion of the object W into the surface layer of the polishing pad 1 as illustrated in FIG. 20C. As shown in FIG. 20B, the this angle .theta.of tilting is actually determined by the action of the sphere 6 according to a relation between the pressing force N and the frictional force f.
As shown in FIG. 20D, tilting of the top ring 3 produces a non-uniform distribution of polish surface pressure so as to cause the pressure at the edge portion to be higher than in the rest of the object W. Because the object W is also rotated, the removal rate distribution becomes one that is illustrated in a graph shown in FIG. 20E. The polish surface pressure to obtain a uniform material removal rate across the object surface of the object W, is also influenced by the softness of the pad and the flow conditions of the polishing solution. Therefore, angle .theta. should not nessarily be equal to zero. However, the angle .theta. which is determined as the result of reaction of the sphere 6 to various forces acting on the top ring 3, as mentioned above, is not necessarily the optimum angle which would produce uniform polish surface pressure.
Also, according to the conventional technique, since the tilt angle .theta. is determined by a result of the reaction of the sphere 6, the possibility exists that consistent polishing is not produced due to local variations in the surface conditions existing at the polish surface of the polishing pad, which may lead to vibration of the top ring to cause further unsteadiness. As a result, as shown in FIG. 20E, more material is removed from the peripheral region than in the central region of the wafer.
Also, in recent years, an alternative type of apparatus has been proposed, which comprises a cylindrical rotating drum having an outer abrading surface. The apparatus applies a line pressure on the wafer, provided by a line contact of the outer periphery of the cylindrical rotating drum with the polished surface of the wafer. While the drum and the wafer are made to undergo relative movement, a polishing solution is supplied to the contacting surfaces to produce a mirror polished surface.
Such rotating drum type apparatus enables the use of more compact polishing tools, compared with the turntable type apparatus, and as a result, the polishing apparatus can also be made more compact. Also, because this approach makes it possible to directly observe the condition of the surface being polished, the amount of material removed or the film thickness remaining on the wafer being polished can be determined on a real time basis.
In the drum type apparatus, the workpiece and the polishing tool are moved relative to each other during the polishing process so as to obtain uniform material removal over the entire object surface of the workpiece. However, the contact length between the polishing tool and the workpiece can vary while undergoing such relative movement; for example, in polishing a circular object such as a semiconductor wafer, the pressing pressure becomes higher in the outer peripheral region of the wafer, resulting in an increased removal rate, to cause a so-called "turned-down edge" phenomenon.
This phenomenon is caused by the fact that the apparatus is operated under a constant pressing load. Also, pressing devices, such as fluid operated cylinders, are not suitable for controlling the pressing pressure because of insufficient response speed and precision due to the presence of fluid pressure adjusting valves, the volumetric effect of the cylinder device and the sliding parts between the cylinder piston and the pressure seals.